Solid lubricant coatings and coating compositions



United States Patent 3,158,45 S9113) LUBRICANT CGA'HNGS AND COATINGCQMPOSITIONS Sylvester F. Murray, Schenectady, and Marshall B. Peterson,Ballston Lake, N.Y., assiguors to General Electric Company, acorporation of New York No Drawing. Filed Sept. 30, 1960, Ser. No.59,492 1 Claim. (Cl. 11746) This invention pertains to solid lubricantcoatings which are efiective to reduce the coefiicient of frictionbetween sliding surfaces at temperatures up to about 1200 F. Moreparticularly, this invention pertains to vitreous lead oxide-silicalubricant coatings, the coating composition from which the solidvitreous coatings are obtained, the method of preparation andapplication of these compositions and the use of the solid vitreouscoatings at the elevated temperatures indicated.

It is known to mix lead oxide and silica in solid form and tosubsequently fuse a coating of the mixture to form a solid lubricantcoating which has been found'useful to reduce the coefiicient offriction between sliding surfaces at temperatures up to about 1200 F.The known coat ings have been bonded generally to low carbon andstainless steel parts and have been found generally effective to reducefriction and surface damage of the coated parts to the elevatedtemperatures. The method for bonding of the known coatings to a metallicsubstrate comprises first pre-oxidation of the metal substrate in airwith heat until a thin yellow and blue oxide film has been formed,thereafter applying the dry powdered coating mixture, and finally firingthe coated substrate at temperatures of approximately 1650 F. to formthe vitreous coating. Although the coatings described satisfactorilyreduce the coetficient of friction between sliding metal parts atelevated temperatures, many problems are encountered with theapplication of these solid compositions to the substrate.

Utilizing the known process above described, it has been found that thepre-oxidation step for preparation of the metal substrates is one ofextreme criticality. More specifically, where the pre-oxidation of themetal substrate was insufficient it was found that subsequently appliedcoatings from a solid mixture of the oxides would not adhere to theoxidized surface. On the other hand, if the metal substrate had beenoverly oxidized, the final vitreous lubricant coating would thereafterspall at the elevated temperature for use of these coatings. Anotherimportant problem associated with the application of a dry powderedmixture to a surface is the difi'iculty encountered With the depositionof the mixture to other than horizontal surfaces. For example, it isoften desirable to coat certain circular bearing surfaces with thevitreous lubricant coatings. Great difiiculty is experienced with theinitial deposition of the powdered mixture of oxides by reason of thepoor adhesive characteristics thereof. The addition of adhesive agentsindis criminately to the powdered mixture generally results in onlycontaminating the subsequently produced vitreous coating and alsoincreases the cost of the coating com-' position.

It has been discovered by the applicants that certain liquid coatingcompositions can be prepared which eliminate or greatly minimize all ofthe above outlined problems. These liquid coating compositions comprisea mixture of a lead compound and a sufiicient quantity of a liquidpolysiloxane adhesive to yield the vitreous coating at the elevatedtemperatures employed to fire the coating. At these firing temperaturesthe lead compound and polysiloxane components of the coating compositionare decomposed to form lead oxide and silica respectively and theseproducts thereafter produce the vitreous lubriice cant composition. Ithas been found that the polysiloxane component in the composition notonly provides an adherent coating so that the coated specimen can be handied and even stored without difficulty before firing, but alsosurprisingly inhibits oxidation of the metal substrate during thefiring. The prevention of excessive oxidation to the metal substrate inturn improves the adherence of the vitreous lubricant coating on thefinal product. The invention may be practiced in its preferredembodiment as illustrated in the following examples and subsequentdiscussion thereon, but it is not limited thereto. Where parts andpercentages appear hereinafter in the specification and claims, they areparts and percentages by weight unless otherwise specified.

Example 1 A stainless steel journal bearing was first cleaned byimmersion for approximately 10 minutes in a hot concentrated liquidsolution of an alkaline phosphate and thereafter rinsed with Water anddried. The bearing was next pre-oxidized to a light yellow color byheating in air to a temperature of approximately 1615 F. forapproximately one minute. The bearing surface was next paintedmanuallywith an approximately two mill thick film of a mixturecomprising grams of lead oxide with 15 grams of the commercial mixtureof a 50% solid solution of methylphenylpolysiloxane in xylene, whichpolysiloxane contained an average of 1.5-1.7 combined methyl and phenylgroups per silicon atom. The coating was thereafter baked for two hoursat approximately 500 F. to cure the polysiloxane and remove the solvent.Next, the coated journal hearing was then placed in a furnace heated toapproximately 1650 F. for from two to three minutes, whereupon the curedcoating had cornpletely decomposed'to form a vitreous lead oxide-silicacoating. The fired bearing was removed from the furmace and cooledrapidly to room temperature. a

In order to measure the effectiveness of the vitreous lead oxide-silicacoating as a solid lubricant for the bearing, friction tests wereconducted on the coated bearing using apparatus especially designed forthe purpose. The means by which friction was measured with thisapparatus consisted essentially of measuring the amount of frictionbetween the end of a rod being rubbed against the coating surface in aplanar manner. More specifically, friction was measured by reciprocatingthe rod against the coated surface in a direction perpendicular to thecircular periphery of the bearing. The rod specimen was mounted in alever arm, so mounted that the free end of the arm was able to moveeither in a vertical or horizontal plane. Various loadings were obtainedby fastening dead weights to the free end of the arm. Sliding wasobtained between the rod and the samples by mov ing the loaded end ofthe arm back and forth manually and the friction generated therefrom wasmeasured in the conventional manner. Heat was supplied to the apparatusin order to measure the effectiveness of the vitreous lead oxide-silicasolid lubricantat elevated temperatures by means of resistance heaterswhich surrounded the'coated hearing. A refractory cover also surroundedthe coated bearing in order to minimize heat loss and to insure uniformheating conditions. The temperature was measured by a thermocouplewhichwas spring loaded against the coated hearing. The coefiicient offriction for theabove coating measured at a temperature of 1100 F. and aload of 17,000 psi. on the arm was approximately 0.15. In contrastthereto, the coefficient of friction for a lead oxide-silica'coatingprepared from a solid mixture of the oxides in the same proportion asthe example ranged from 0.125 to 0.20.

As'a further test of the effectiveness of the present coatings atelevated temperatures, the endurance life of the coating was determinedusing the above apparatus. The endurance test was conducted by movingthe loaded arm back and forth with an air cylinder for a specific numberof cycles. The coefiicient of friction for the coating of the exampleafter 1,000 cycles at a velocity of travel of the rod of approximately3.7 feet per minute was found unchanged.

Example 2 A low carbon steel bearing coated according to the method ofExample 1 except that for the liquid coating composition employedtherein, there was substituted a mixture comprising 95 parts lead oxidewith 30 parts of the organic polysiloxane solution used in that example.The coeflicient of friction measurements made upon the solid vitreouscoating obtained by curing and firing the polysiloxane coating in themanner hereinbefore described gave results substantially comparable tothe results obtained with the coating of Example 1.

Example 3 To still further illustrate the use of compositions within thecontemplation of the invention, a stainless steel bearing was coatedaccording to the method of Example 1 except that for the liquid coatingcomposition employed therein, there was substituted a mixture comprising95 parts lead oxide with six parts of the organic polysiloxane solutionused in that example. The coefficient of friction for the vitreous leadoxide-silica coating obtained by curing and firing the polysiloxanecoating in the manner hereiubefore described was substantiallycomparable to the results obtained with the coating of Example 1.

The liquid coating compositions of the present invention are mixtures ofa lead compound and a liquid polysiloxane material which reacts atelevated temperatures in air to form a vitreous lead oxide-silicaproduct. These compositions may be characterized as being storage stableat ordinary temperatures and non-reactive with a low carbon steel orstainless steel material. It is not deemed critical to the successfulpractice of the invention that the lead compound be dissolved in theliquid coating compo sition, and successful results have been achievedwith lead oxide suspensions in the liquid polysiloxane adhesive.Further, it is not believed that the solids content of the liquidcoating compositions of the invention is critical, since there are manyknown ways to achieve a final solid coating of desired thickness from acoating composition having a givensolids concentration. For example, ifthe coating composition has comparatively low solids concentration, itis still possible to obtain comparatively thick films simply by multipleapplications of the coating composition. On the other hand, where it isdesired to obtain comparatively thin coatings from a high solids coatingcomposition, excess liquid coating canbe removed after application .tothe substrate by means of a roller, doctor knife, or other known means.

The ratio of lead compound to liquid silicone adhesive in the coatingcomposition is important in order to obtain the final vitreous coatingwhich constitutes the solid lubricant film of the invention. Morespecifically, it is necessary that the ratio of lead compound topolysiloxane in the liquid coating be such that, upon firing of the applied coating at the temperatures indicated in air, a vitreous coatingof lead oxide-silica be formed. In other Words, the polysiloxane shouldfurnish sufficient silica to thereafter combine with the lead oxideformed in such proportions to form a solid vitreous coating. It isfurther necessary that the proportions of polysiloxane and lead compoundbe such to form the vitreous coating upon firing in air, which vitreouscoating will be a solid at temperatures up to about 1200 F. The need fora solid vitreous coating at elevated temperatures is to maintain thelubricant on the bearing surface during its use and thereby extend thelife of the lubricated bearing. It is obvious that a lubricant whichwould be a liquid at especially at elevated temperatures.

from the bearing surface and thereby increase wear of the bearing. Theratio of lead compound to polysiloxane which has been found'useful toprovide a solid vitreous coating can best be characterized as that whichwill yield from two to 22 parts silica and 98 to 78 parts lead oxide.The preferred ratio of silica to lead oxide in the vitreous coating istwo to 10 parts silica and 98 to 90 parts lead oxide for the reason thatoptimum low friction and long life characteristics of the coatedsubstrates are obtained in this range. 7

The solid vitreous coatings of the invention can best be characterizedby uniformly low friction characteristics and low wear of the coatedproduct atelevated temperatures up to about 1200" F. In addition, thecoatings impart greater oxidation resistance to the coated surfaceAlthough the thickness of the coating has not been found critical toobtain the improved properties described, it is obvious that for use onbearing surfaces under sufficiently heavy load that a maximum thicknesswill be dictated above which some of the coating can be removed duringuse. Coatings up to three mils thickness are preferred to obtainimproved life of the coated bearing without suffering loss of thecoating due to the effect described. On the other hand, improved life ofthe coated hearing has been obtained with coatings having a thickness ofbut one mil. It is, therefore, obvious that the optimum thickness of thevitreous coating will be a function of the particular application forthe coated surface and will be affected by such considerations as load,temperature, and even velocity of the moving coated surfaces.

The lead compounds which are useful in the practice of the invention canbe selected from the class of organic and inorganic compounds of leadwhich can be. oxidized at elevated temperatures and the presence of asilicone adhesive to form lead monoxide (PbO), the lead monoxidethereaftcr not converting to oxide (Pb O at the elevated temperatures ofuse. The reason for distinguishing between the various lead oxides isthat the latter mentioned oxide has been found to destroy thelubricating properties of the vitreous lubricant coating. Useful leadcompounds may be further characterized as those which in the formapplied inthe liquid composition will not react deleteriously with thesubstrate. For example, those lead compounds which are the salts ofstrong acids such as lead chloride and lead sulphate are believedunsatisfactory by reason of reacting at elevated temperatures toliberate acidic products from the coating compositions which will attackthe substrate. Useful lead compounds include lead carbonate, leadsilicate, lead formate, and lead naphthenate. Lead oxide is thepreferred lead compound by reason of not requiring conversion atelevated temperatures which would result in the formation ofby-products, such by-products although perhaps not acting deleteriouslywith the substrate could still act as a diluent in the final leadoxide-silica composition.

Useful polysiloxane adhesives comprise polysiloxane compounds which areeither adhesive liquids at ordinary temperatures or can be dissolved inorganic liquid solvents to form liquid adhesive coating compositions.The useful polysiloxane compounds can further be characterized by anabsence of volatilization before conversion to silica at the elevatedfiring temperatures. Suitable polysiloxane adhesives can be selectedfrom the well-known methylsilicone resins, high molecular weightsilicone oils, and polysiloxane elastomers. Polysiloxanes having anaverage 1.2 to 1.7 combined aliphatic and aromatic groups per siliconatom are preferred by reason of being heat curable to form a solidnon-tacky film that permits handling of the coated object without needfor immediate firingof the coating to 1650 F.

The class of utilizable polysiloxane compounds described above isunderstandably a broad one and includes alkyl Silicone polymers, arylsilicone polymers, alkyldensation products composed essentially ofsilicon, oxygen, and at least one methyl group attached directly tosilicon. The methyl group or groups may be attached to any or all(preferably to all) of the silicon atoms contained in the molecule.Ordinarily, the polymer will have in its molecule an average of one toapproximately two methyl groups per silicon atoms. Typical aikylsilicone polymers also include compositions composed essentially ofoxygen atoms and ethyl radicals each bonded to silicon atoms and whereinthe average ratio of ethyl radicals per silicon atom is from 0.5 to 1.5.Useful aryl silicone polymers include dipheny-l silicone and siliconeswhich are the reaction product of such alkyl benzene starting materialsas toluene, xylenes, mono-, di-, and triethyl benzenes. cone polymerswhich are useful include halogenated products such asdimethyldichlorosilane, methyltrichlorosilane, methylchlorophenylsilicone, fiuorophenyl silicone, and aroxyalkyl or aroxyaryl siliconessuch as methyl phenoxy phenyl silicone and di-(phenoxy phenyl) silicone.

It is not intended to limit the coating compositions of the invention toa mixture of a lead compound and a polysiloxane adhesive merely. Forexample, as has been shown in the above examples, it is preferred thatthe coating composition contains a sufiicient quantity of an organicliquid solvent to maintain a comparatively low viscosity for the coatingcomposition. One advantage obtained with a low viscosity coatingcomposition is an ability to coat more irregular shapes at ordinarytemperatures than is possible with a higher viscosity coatingcomposition. It is also possible to incorporate other inert agents inthe coating composition, such as suspending agents, extenders, etc., ifdesirable, Without altering the nature or" the final vitreous coating.For example, the addition of many organic compounds can be expected toresult in the absence of any residue therefrom by reason of the elevatedtemperatures employed in the firing operation.

Typical substituted products of the sili- 6 It is also notintended tolimit the method of obtaining the final vitreous lead oxide-silicacoating to the means disclosed in the above examples. For example, itwill be obvious that more elevated firing temperatures may be employedto .form the vitreous coating if needed, since it has been shown thatthe polysiloxane component or" the coating composition is effective toimprove the oxidation resistance of the substrate. On the other hand,firing temperatures as low as approximately 1300 F. have been foundadequate to produce the vitreous lead oxide-silica coating. Also, it isobviously not intended to limit the means for application of the liquidcoating composition to the method of manual painting illustrated in theabove examples. Other'known means, including spray, roller, knife, anddip coating, are suitable for the application of the liquid coatingcomposition to the substrate.

It is, therefore, intended to limit the invention only by the scope ofthe following claim.

What We claim as new and desire to secure by Letters Patent of theUnited States is:

A method for providing a bearing lubricant to a substrate whichcomprises coating the substrate with amixture containing polysiloxaneadhesive and an oxidizable non-corrosive solid lead compound in ratiosyielding 2 parts si ica and 98 parts lead oxide to 22 parts silica and78 parts lead oxide and heating the coating in air at temperature of atleast 1300 F. for a sufficient time to form a vitreous product of leadoxide and silica.

Referenees titted in the file of this patent UNITED STATES PATENTS2,198,253 Koehring Apr. 23,. 1940 2,593,817 Waggoner Apr. 22, 19522,748,030 Silversher May 29, 1956 FOREIGN PATENTS 749,641 Great BritainMay 30, 1956 OTHER REFERENCES Johnson et al.: Lubrication Engineering,vol. 15, No. 12 (December 1959) (pp. 487-91 and 496).

